The present invention generally relates to a control system for an internal combustion engine and more particularly, to an ignition timing control system for an internal combustion engine equipped with a supercharger.
Recently, in an internal combustion engine equipped with an electronic control system, an ignition timing is generally controlled momentarily in compliance with running conditions of the engine, with a view to preventing the knocking, to raising an engine output and to preventing a change for the worse of emission such as exhaust emission or the like.
The knocking is liable to take place, as a temperature within a combustion chamber or chambers increases. When the knocking has taken place, a thermal load within the combustion chamber increases, thus inviting a drop of the engine output, thermal damage of some parts or the like, and a noise caused by vibration is remarkably increased.
The knocking has a close connection with the ignition timing. When the ignition timing is advanced, the knocking is liable to arise, since the temperature within the combustion chamber increases due to improvement of the combustion efficiency. On the contrary, when the ignition timing is delayed, the knocking hardly arises, since the temperature and pressure within the combustion chamber are lowered.
Although it is well known that the knocking is liable to arise as an intake air temperature becomes higher, the knocking is hard to occur as the intake air becomes higher in humidity.
It is also known that the knocking is hard to take place in summer rather than in winter under the influence of density or humidity of the ambient air, notwithstanding the fact that the temperature thereof is generally high in summer.
One of the ignition timing control systems of the engine is disclosed, for example, in Japanese Patent Laid-open Application (Tokkaisho) No. 58-202374, in which the ignition timing is advanced by a prescribed angle in summer rather than in winter, in view of the fact that the knocking is hard to arise in summer.
In the case where the engine is equipped with a supercharger, the knocking is liable to arise due to improvement of the combustion characteristics following a rise of the intake air temperature caused by the boosting and improvement of the charging efficiency.
Japanese Patent Laid-open Application (Tokkaisho) No. 58-5471 also discloses the ignition timing control system of the engine which is capable of correcting the ignition timing towards the side of spark retardation, with a view to preventing the knocking at the time when the intake air temperature at the downstream side of the supercharger exceeds a predetermined value.
In the engine equipped with the supercharger, it is not uncommon, to further heighten the charging efficiency, that an inter-cooler is generally provided at the downstream side of the supercharger so that the intake air raised both in pressure and in temperature by the boosting may be cooled by the inter-cooler. In the case where the supercharger is a turbocharger, the boosting capability varies remarkably in connection with an engine speed. Furthermore, in the case where the inter-cooler is of an air cooled one, the cooling capability thereof also considerably varies.
Accordingly, upon detection of an amount increased in the charging efficiency caused by the boosting, it has been extremely difficult to correct the ignition timing towards the side of spark retardation in compliance with the amount increased in the charging efficiency.
In the ignition timing control system disclosed in the above-mentioned Application No. 58-202374, since the ignition timing is controlled on the basis of the temperature of the intake air i.e., the ambient air drawn into an intake air passage, it is possible to execute the control, for example, for advancing the ignition timing by a prescribed angle in compliance with atmospheric conditions in summer. This kind of ignition timing control system, however, includes a disadvantage in that the ignition timing is not accurately controlled in accordance with an amount of temperature variation of the intake air caused by the supercharger and the inter-cooler, even if this kind of ignition timing control system is applied to the engine equipped with the supercharger and the inter-cooler. Furthermore, it is actually impossible to detect the conditions such as the pressure, temperature or the like of the intake air drawn into the combustion chamber only through detection of the ambient temperature by an intake air thermo-sensor disposed in the vicinity of an air flowmeter. Accordingly, it is substantially impossible to correct the ignition timing in view of the influence of the boosting.
Moreover, the ignition timing control system disclosed in the aforementioned Application No. 58-5471 is of the type employed in the engine equipped with the supercharger and has been developed merely on the basis of an idea such that the ignition timing is corrected by an amount increased in the intake air temperature after the boosting. In addition, since the ignition timing is controlled in compliance with the intake air temperature at the downstream side of the supercharger, it is disadvantageously impossible to accurately correct the ignition timing in accordance with the atmospheric conditions.